Martensitic steel



P \om 2 Q1 6 69 10$ INVENTOR.

Charles M. Hammond ma ATTORNEY United States Patent 1 3,251,683 MARTENSTTIC STEEL Charles M. Hammond, Sarver, Pa, assignor to Allegheny Ludlum Steel Corporation, Brackenridge, Pa, 21 corporation of Pennsylvania Filed Jan. 16, 1962, Ser. No. 166,510 Claims. (Cl. 75128) This invention relates to improvements in age hardenable stainless steels, and in particular to martensitic stainless steels which may be age hardened to a hardness ranging between 41R and 54 R Different stainless steels have been recently developed in order to meet the requirements of manufacturers of aircraft and especially aircraft designed for flight at supersonic speeds.- These supersonic aircraft are often referred to as hot aircraft, since during flight at speeds greater than Mach 1, the surfaces of the aircraft become heated. As' a result, particular attention has been given to the material from which such aircraft are manufactured so that the material will exhibit strength characteristics at temperatures of up to 1000 F. In other applications, the temperature range may be extended downwardly to temperatures considerably below -l00 F., for example in missiles; thus the effective range of these materials may run the gamut from the so-called cryogenic temperatures to those suitable for the hot airplanes. This has led the material suppliers to develop steels and alloys having a wide range of useful temperatures; In substantially all of these applications an additional requisite must be present in the material, that being the aspect of exhibiting a good resistance to corrosion. Consequently, stainless steels have occupied a position of considerable interest within the last few years. As a result thereof, the metal manufacturers have developed a family of stainless steels which are intermediate the conventional AISI Type 300 Series and the AISI Type 400 Series.

Most of the steels which are presently in use are austenitic stainless steels which, through various forms of subzero cooling and/or double aging, effect an adjustment of the internal chemical composition so that the steel which is normally austenitic at room temperature may be made martensitic in order to obtain the strength advantage of the martensitic structure. In addition, some metal manufacturers have also superimposed an age hardening mechanism in addition to the martensitic mecha nism in order to obtain a high degree of strength in the steels. As such, the austenitic metals known to the trade under the names of AM-350, AM 355, 17-7 PH and PH -7 Mo have exhibited an austenitic structure at room temperature which makes the steel amenable to various forms of fabrication, and the thus-fabricated part may thereafter be heat treated to obtain a variation in the internal chemistry of the steel so that upon cooling to room temperature or to some predetermined subzero temperature, the steel from which the article has been manufactured may be transformed to martensite in order to. obtain the requisite strength characteristics within the steel. Thereafter the article may be subjected to a low temperature heat treatment to effect a response to tempering and/ or aging in order to obtain optimum strength in the material. However there exists a fundamental difficulty with these so-called austenitic stainless steels in that during severe forming operations the M temperature may be traversed so that the steel partially transforms to martensite during the forming operation, thus seriously inhibiting the formability of the steel.

Other compositions have also been derived which normally have a duplex microstructure in the annealed condition at room temperature consisting of islands of delta ice ferrite in a martensitic matrix. This resulted from a particular balance of the chemical components and thus, while these steels exhibit good formability, the strength levels which can be obtained leave a great deal to be desired. In addition, the presence of delta ferrite within the microstructure conferred a high degree of directionality to the mechanical properties of these steels, with the result that their use has been fairly limited, especially where the basic material has undergone even a moderate amount of cold reduction. In order to alleviate the prior art difiiculties, a low carbon martensitic stainless steel has now been produced by me which possesses a high degree of formability and which may be age hardened in order to provide a high degree of strength therein.

An object of the present invention is to provide a martensitic, age hardenable stainless steel which is suitable for use at temperatures below 1000 F.

Another object of this invention is to provide a martensitic stainless steel which exhibits a high response to an age hardening heat treatment.

A further object of this invention is to provide 21 martensitic age hardenable stainless steel which exhibits high strength characteristics and which may be readily formed when the steel is in its completely martensitic condition resulting from either cold working or a subzero cooling at a temperature of no lower than -l00 F.

A more specific object of the present invention is to provide a martensitic stainless steel which is amenable to an age hardening heat treatment resulting from a critical balance of the austenite-forming elements and the ferriteforming elements and which contains, as essential alloying elements, carbon, chromium, nickel, cobalt, molybdenum and titanium.

These and other objects of the present invention will become apparent when taken in conjunction with the following description and the accompanying drawings in which:

The figure of the accompanying drawing is a modified Schaefiler diagram which illustrates the range of composition for balancing the austenite and ferrite formers into which the steel of the present invention must fall.

In its broader aspects, the steel of the present invention contemplates a composition which includes up to 0.05% carbon, up to 0.25% manganese, up to 0.25% silicon, from about 10% to about 18% chromium, from about 0.1% to about 7% nickel, from about 3% to about 16% cobalt, from about 0.1% to about 8% molyb denum, from about 0.1% toabout 1.3% titanium and the balance substantially iron with incidental impurities.

The steel of the present invention may be made in any of the well-known manners, the details of which will not be described in any extensive manner herein. After the steel of the required composition is melted, it may be cast into ingots which are thereafter hot rolled into the form of a semi-finished mill product. Thereafter the steel may be cold rolled to the desired mill product and given any variety of heat treatment which will be most desirable from the fabricators point of view.

Each of the alloying elements present within the steel of the present invention performs a specific function, and thus must be limited to Within the ranges statedhereinbefore. Carbon is preferably present in an amount of up to 0.05%, and is a strong austenite-forming element. While the carbon content is preferred to be kept low, since it is known that the amount of carbon within the martensitic phase is the most influential element in determining the hardness of martensitic structure of steel, it has been found that the present invention can utilize carbon in an amount of up to 005%. Because the normal structure of the steel is martensitic in the annealed Patented May 17, 1966 condition at room temperature, and since it is preferable in the steel of the present invention to have a low carbon martensite phase, this amount of carbon will not unduly harden the steel so as to impair the formability characteristics exhibited .by the steel. Optimum formability, however, appears to be obtained when the carbon content is limited to about 0.03% maximum. As thus contrasted to the prior art steels which utilize carbon in order to obtain the high strength characteristics in the martensitic phase, the present steel does not derive its optimum mechanical properties from the martensitic condition of the steel. In addition, these low amounts of carbon are extremely advantageous from the standpoint of corrosion resistance since less chromium carbides are formed, thereby increasing the relative amount of chromium available for corrosion resistance purposes. In addition, carbon, which is exceedingly potent in regulating the level of the M temperature and the M temperature, is preferably maintained at a low level, thus permitting the utilization of other elements for this function without detrimentally aifecting the ultimate obtainable strength characteristics in the steel.

Resistance to corrosion is imparted to the steel of the present alloy through the presence of chromium. In addition, chromium is a strong ferrite-forming element and thus must be critically balanced against the austeniteforming elements. In this respect, a compromise between ferrite formation and corrosion resistance may bestruck wherein the chromium content may be as loW as On the other hand, if the chromium content exceeds about 18%, the alloy does not exhibit an adequate response to age hardening. Moreover, the steel with amounts of chromium in excess of 18% may form delta ferrite depending upon the balance of the other alloying elements present which, if formed, is extremely detrimental to the mechanical properties exhibited by the steel, both from the aspect of the relative level of the obtainable strength and from the aspect of conferring directionality to the properties. The optimum-combination of corrosion, resistance, stability of chemical composition and age hardening response appears to be obtained when the chromium content is maintained within the range between about 12% and about 15%. This chromium content will not unduly depress the M temperature of these steels, the factor for chromium being about '50 F. degrees per percent element when taken into solution, yet the steel will possess corrosion resistance and exhibit a good response to age hardening.

Cobalt appears to be the most critical element within the present steel. In this respect, cobalt enters into the age hardening response forming what is believed to be a complex cobalt-nickel-molalbdenum-titanium precipitate which is coherent with the basic lattice structure, thus providing the steel with an outstanding combination of mechanical properties. Moreover, cobalt has been determined to be an austenite-forming element, and it is almost as potent as nickel, the nickel equivalent of cobalt being about 0.7 times the percent cob-alt; however, the eifect of cobalt on the depression of the M temperature of the steel of the present invention is only 8 F. degrees per percent cobalt as compared with about 70 F. degrees per percent nickel. Consequently, cobalt is one of the most important elements in balancing the composition of the steel of the present invention. Moreover, cobalt is also eifective in maintaining the high temperature strength of the alloy so that the steel can be used at temperatures of up to about 1000 F. In this respect, it has been found that at least 3% cobalt is necessary in the steel of the present invention in order to obtain anadequate response to the age hardening treatment and for the proper balance of the austenite-forming elements with the ferriteforming elements. It is believed that cobalt, in excess of about 16%, while being effective for balancing the composition and providing a response to the age hardening heat treatment, may increase the annealed hardness of the steels so that in the annealed condition the steel may exhibit a sufficiently high hardness, which may impair the formability of the steel. The optimum cobalt content is within the range between about 10% and 16% wherein the steel exhibits the proper M temperature and response to age hardening without adversely affecting the formability of the steel by unduly hardening the steel in the annealed condition. "It should be pointed out that where it is desirable to sacrifice part of the strength characteristics of the steel of the present invention in order to impart to the steel a high degree of resistance to stress corrosion, the steel of the present invention will preferably have a cobalt content between about 3% and about 7%. When the cobalt content .is maintained within this low range, the steel must be balanced as respects the austenite-forming elements and the ferrite-forming elements with an increase in the nickel content to maintain a substantially completely martensitic structure within the steel of the presentinvention. As a result thereof, the steel does not provide as high a degree of response to the age hardening heat treatment; however, the strength level is more than adequate for most applications. These low amounts of cobalt appear to be necessary where it is desirable to have the maximum amount of resistance to stress corrosion at the expense of the strength characteristics.

Nickel is present within the alloy in an amount between 0.01% and about 7% since it has been found that nickel enters into the age hardening response of the subject steel and is effective for regulating the M as well as the M temperature of the steel. In addition, nickel also is an austenite-forming element and is thus balanced against the ferrite-forming elements so as to obtain a completely martensitic structure within the steel in the annealed condition at room temperature. 7% decreases the M temperature to a sufficient degree that the steel will not transform to martensite upon cooling, even to temperatures below about 100 F. Consequently, the steels will not age harden to develop the proper strength characteristics. The optimum results as respects the M and M temperatures, as well as the balancing of the austenitic components against the ferritic components, appear to be obtained when the nickel content is maintained within the range between about 2% and 7%. Within this range the steel will exhibit the proper M -and M temperatures and will provide an optimum response to the age hardening treatment, thereby developing the optimum mechanical properties within the alloy.

Molybdenum is present within the steel of the present invention within the range between about 0.1% and about 8%. At least 0.1% molybdenum is necessary to enter into the precipitate which is formed to enhance the mechanical properties of the steel. Molybdenum contents in excess of about 8% unduly depress the M and M temperatures and are effective for the formation of delta ferrite which will unduly decrease the strength attainable by the steel and will confer directionality upon the properties. It is also to be noted that molybdenum is highly effective for forming an excess phase believed to be chi phase which may adversely affect the properties of the present steel. Thus, where the steel is to be used in an application where it will be subjected to a heat treatment, as for example a welding or brazing cycle, deleterious amounts of chi phase may be formed .where the molybdenum content exceeds about 4%. Thus the end application will, to some extent, determine the maximum optimum level of the molybdenumcontent. On the other hand, where the steel will be used in an application which will not be subjected to a high temperature heat treatment, additional amounts of molyb denum, namely between 4 and about 8%, may be utilized since any chi phase which is formed can be readily eliminated by heat treatment. Molybdenum also acts to provide an increased response to corrosion resistance,

Nickel in excess of about 7 boron up to 0.10% or zirconium up to 0.10%.

especially in atmospheres containing any of the halogen elements. The optimum combination between strength characteristics, response to age hardening, corrosion resistance and balanced chemical composition appears to be imparted to the steel of the present invention when the molybdenum content is maintained in the range between about 1% and about 6%.

Titanium is included within the composition of the subject steel and is present within the range between 0.1% and 1.3% for its effect in entering into the precipitation hardening component, as well as its aifinity for carbon, thus minimizing the amount of carbon available for forming a chromium carbide and increasing the corrosion resistance of the alloy. Titanium contents in excess of about 1.3% do not appear to be effective for increasing the hardness of the-steel and may detrimentally affect the steel through the formation of compounds which will render the alloy notch sensitive. Titanium is also effective for depressing the M temperature, and accordingly must be limited since research has indicated that in the subject composition the effect of titanium is to depress the M temperature about 100 F. degrees per percent of titanium. Accordingly, the titanium content must be limited to within the range between about 0.l% and about 1.3%. The optimum combination of mechanical properties in response to age hardening, M and M temperature, and delta ferrite formation appears to be obtained when the titanium content is maintained within the range between 0.1% and The alloy of the present invention may alsooptionally contain such elements as calcium up to 0.10%, The balance of the alloy is essentially all iron with incidental impurities such as is normally found in commercial steel mill melting practices.

Reference is directed to Table I'which sets forth the chemical composition of the general range and the preferred range of each of the elements which essentially comprise the composition of the steel of this invention:

As was stated hereinbefore, the steel of the present invention must be balanced in order to obtain the full benefits attainable within the steel. In this respect, it is imperative that the steel maintain a balance between the austenite-forming elements and the ferrite-forming elements. To this end, referencemay be had to the figure of the drawing which is a modified Schaefller diagram which has been derived to more closely approximate the equilibrium condition of these steels. In this respect, it should be noted that the original Schaeffier diagram is based upon cast weld materials, whereas the present modification has been derived for wrought materials. Therefore, the zero percent delta ferrite line which appears in the drawing is displaced downwardly from its corresponding position in the original Schaefi'ler diagram. Thus the pseudo-phase diagram of the drawing illustrates the relationship between the austeniteforming elements, expressed as nickel equivalents vs. the

ferrite-forming elements, expressed as the chromium equivalents. As expressed therein,

Ni eq.=30(percent C+ percent N) percent Ni+0.5% Mn+0.7% Co and the relationship between the chromium and chromium equivalents is expressed as- Cr eq.: percent Cr+1.5% Si percent Mo+1.5% Ti It will be appreciated that the nickel equivalents express the relationship of the relative power of the austenitizing elements and the effect upon the microstructure of the steel, whereas the chromium equivalents express the relationship of the power of these elements and their fer ritizing effect upon the structure of the steel. By inspection of the drawing it is seen that a steel having a composition within the limits set forth hereinbefore in Table I and which has a chromium equivalent and a nickel equivalent as computed in accordance with the equations given hereinbefore will fall within the limits of the pseudo-phase diagram of the drawing. However, it is only the alloys which have a balanced composition of nickel equivalents and chromium equivalents which fall generally within the portion designated AcBCDaA which Will respond to provide a substantially completely martensitic structure and which is amenable to an age hardening heat treatment, to be set forth more fully hereinafter, to provide the steel with the optimum combination of formability, attainable mechanical properties and corrosion resistance. Thus, the steels which fall within area AcbaA of the drawing will transform to martensite upon cooling to a temperature no lower than F. The completely martensitic structure exhibits a hardness of less than about 35 R Alternatively, the composition is also balanced so that through the application of between 25% and 50% cold work to the .steel, the M temperature will be traversed with the result that the alloy will exhibit a completely martensitic structure after the application of cold work. Referring again to the drawing, steels having a composition which, when balanced in accordance with the nickel equivalents and chromium equivalents set forth hereinbefore so as to have a composition lying within the area bounded by BCDaboB of the drawing will require the application of up to 50% cold work in order to obtain a completely martensitic structure in these steels. Apparently this results from the fact that while a partial transformation of aus'tenite to martensite may be accomplished through a cooling to a temperature of no lower than 100' R, a completely martensitic structure will not be obtained. However, the application of up to 50% cold work is usually sufficient for obtaining a completely martensitic structure. Since the steel of the present invention has an extremely low rate of work hardening, the application of this amount of cold work will not unduly harden the steel and thus will not detract from the formability of the steel. The application of the cold Work is done at room temperature, which is sufficiently below the M temperature to assure a completely martensitic structure. Thereafter, the steel may be subjected to the same aging treatment for substantially the same period of time and the steel will exhibit an age hardening response at sub stantially the same level as the steels which have a balanced composition so as to fall into the area AcbaA of the drawing. Where desired, it is also possible to combine the effects of both temperature and cold work, either concurrently or independently, and thus it may only be necessary to cold work the steel less than 25% or cool it to a temperature higher than 100 F. in order to obtain the fully. martensitic structure within this steel.

As stated hereinbefore, the steel of the present 'invention obtains its optimum combination of mechanical properties through the function of an aging mechanism.

period of between about 4 and 12 hours.

Thus, it is essential that all of the age hardening components be taken within solution in order to form the age hardening precipitate which is coherent with the lattice structure of the matrix. This is accomplished through the application of an annealing heat treatment,

such annealing heat treatment being accomplished at a temperature within the range between about 1500 F. and about 2100 F. The steel must be held at a temperature within this range for a sufficient length of time in order to insure that all of the age hardening components are taken within solution. 'A temperature of at least 1500 F. must be attained since temperatures below this level will be insufficient to take all of the hardening components within solution. Heating to a temperature above 2100 F. does not appear to be beneficial,

and in some instances may be effective for forming deleterious phases. Where the steel of the present invention has a molybdenum content in excess of 4%, it is usually desirable to heat the steel to a temperature near the upper end of the range, i.e., a temperature in excess of 2000 E, in order to take all of the chi phase within solution. Thereafter, the steel is cooled to a temperature of no lower than -100 F. Thus, the steel having the balanced composition within the area AcbaA will completely transform to martensite upon cooling to said temperature. Metallographic examination and X-ray readings indicate a substantially martensitic structure. On the other hand, where the steel has a balanced composition so as to fall within the area BCDabcB it will require the application of up to 50% cold work or the combination of cold work and cooling to a temperature no lower than l'00 F. for the effective transformation of the austenitic phase to martensite. The steel in the completely martensitic condition and with all of its age hardening components within solution may be thereafter subjected to an aging heat treatment in order to obtain the optimum mechanical properties within the alloy. This aging heat treatment is preferably accomplished by heating the steel to a temperature within the range of about 600 F. and 1100 F. and holding the steel at this temperature range for a period of up to 8 hours or more. Optimum results appear to be obtained when the steel is annealed at a temperature of about 1550 F. to about 1700' F., transformed to martensite and thereafter aged at temperatures between 900 F. and 1000 F. for a time As thus heat treated, the steel possesses its optimum combination of mechanical properties.

In order to more clearly demonstrate some of the TABLE II Chemical analysis (percent by wt.)

Heat 0 Mn Si Or Ni Co Mo Ti Fe 319. 0.019 0.010 0.02 15.50 3.37 15.13 2.11 0.50 Bal. 365 0.008 0. 006 0.02 15.41 15.86 2.05 0.39 Bal. 367 0.008 0.006 0.02 15.46 3.42 15.39 2.07 0.41 Ba]. 368.-. 0.008 0.003 0.02 15.32 3.36 9.99 2.11 0.41 Bal. 369 0.008 0.009 0.02 15.50 3.34 6.99 2.09 0.39 Bal. 370 0.007 0.010 0.01 12. 03 3.42 15.19 2.07 0.39 Bal. 371 0.006 0. 006 0. 02 13.01 3.41 15.48 2.07 0.39 Bal. 373 0.004 0.006 0.01 15.55 6.98 15.53 2.11 0.41 Bal. 452 0.019 0. 007 0. 02 14.09 3.46 15.50 2.03 0.43 Bal. 454 0.006 0.005 0. 02 14.27 3.46 15.57 0. 46 Bal. 456 0.005 0. 005 0.03 13.04 3.50 15.51 2 94 0. 47 Bal. 467 0. 005 0.002 0.01 12.04 3.46 15.86 4.12 0.47 E31.

As stated hereinbefore, the alloying elements which are utilized in the composition of the steel of the present invention must be maintained within the range set forth hereinbefore in Table I. The effect of the elements co- 8 balt, nickel, chromium and molybdenum on the hardness of the steel of the invention is set forth in Table III:

TABLE III Efie ct 0f alloying elements Hardness Hardness Element Percent Heat N 0. after Ann. after Aging F. at 950 6.99 369 28 Rc.. 41.2 Re- 9.99 368 45 R3. 15. 39 367 51.8 R3. 0 365 41.3 Re- 3. 42 367 51.8 R3. 6. 98 373 92.5 Rb 12. 08 370 43.6 R3. 15. 46 367 51.8 R. 18.01 371 22.8 R3. 2. 03 452 51.3 B3. 2.94 456 50.6 R6.- .4. 12 467 52.1 R6.

From examination of the hardness values set forth in Table III, it is readily apparent that the steel of the present invention must maintain the cobalt content within the range between about 3.0% and about 16%. At about 7%, the steel will have a hardness in the martensitic condition of about 28 R Increasing the cobalt content to about 10% is effective for increasing the hardness only to 29 R It is to be noted, however, that when the cobalt content is increased to its upper limit, the martensitic hardness correspondingly increases as set forth in Table 111. It is apparent that after the aging treatment, a great increase in the hardness is noted, the minimum amount being 13 R units. Thus, c0balt must be maintained within the range of about 3.0% and about 16%.

Nickel is effective in the steel of the present invention and reacts somewhat similar to cobalt in its effect upon the hardness of'the steel. However, when the nickel content is increased to more than about 6%, as for exam- .ple in Heat 373 which contains 6.98%, while the steel has a low martensitic hardnessthe same being about 91 R a subsequent aging treatment is ineffective for precipitate will form which is coherent with the lattice structure of the martensitic steel, thus exhibiting an outstanding increase in the hardness.

Chromium also exerts a strong influence on the steel,

it being noted from Table III that increasing the chromium content from about 12% to about 18% is effective for decreasing the hardness and this may be expected from the standpoint that the high amounts of chromium are effective either for the formation of delta ferrite which will not transform during any subsequent cooling or cold rolling operation, or the steel may be sufficiently stabilized so that after cooling to a temperature of no lower than 100 F., the steel will still contain retained austenite. While up to about 18% chromium can be utilized in the steel of the present invention, it is necessary to balance the 18% chromium with other austeniteforming elements so as to maintain a microstructure which is substantially free of delta ferrite and which will transform to martensite upon cooling to room temperature or to 100 F., or in the alternative, which will steel of the present invention. By increasing the molybdenum content from 2.03 up to 4.12%, little effect is shown in the martensitic hardness of the steel, the level being about 32 R however, after aging these steels a 9. great increase of from 18 to 20 R units is noted in the hardening response. It is clear from the foregoing that each of the alloying elements must be maintained within the range set forth hereinbefore in Table I and each of the elements must be so-balanced as to maintain a substantially completely martensitic structure within these steels. The presence. of delta ferrite is highly detrimental, not only to the mechanical properties but also in the response to the age hardening heat treatment.

The steel of the present invention exhibits outstanding tensile properties. Reference is respectfully directed to Table IV which illustrates the test results after various forms of heat treatment on two steels which fall within the range of the present invention.

TABLE IV When the aging time of the steel was increased to 8 hours, a greater increase in the hardness was noted, as well as the corresponding properties of the yield strength and ultimate tensile strength. Moreover, the ratio of the notched tensile strength to the ultimate tensile strength was greater than unity.

The steel of the present invention having a substantially completely martensitic structure may be cold rolled. After cold rolling 25% followed by sub-zero cooling to -100 F. for 16 hours and thereafter air warming, the steel possesses a slightly higher hardness than when transformed directly by thermal treatments. Since the increase in hardness amounted to merely about 2.45 R units, it is apparent that the steel of the present invention has an Hard- 0.2% U.T.S., Elong, NTS/ Heat Treatment ness Y.S., K s.i. percent; UTS R K 5.1

1.-.. 1,600- min, A.C. 100 F.16 32 100. 9 145. 5 8.5

hrs., A.W. 2 (1)+950 F.4 hrs, A.C 47. 2 214.4 219. 5 6 3 (1)+950 F.-8 hrs., A.C 49.1 218. 5 226. 4 8 4 O.R., l00 F., 16 hrs., A.W 34. 7 157. '7. 165. 5 6 5 50% C.R., -100 F., 16 hrs., A.W 36.8 169. 3 177. 0 5 6.-.. (4)+900 F., 1 hr., A.C l. 47 229. 3 229. 3 4.0 7 (4)+900 F., 4 hrs., A.C 48 238. 4 238. 4 3. 5 50 299. 7 250 4.0 48. 7 235.8 235.8 4. 0 s 50.2 241.8 242.6 4. 5 ll... (4)+950 F., 8 hrs., A.O 50. 5 241. 7 245. 4 5. 5 12.-. (5)+950 F., 4 hrs, A.C 51.1 256. 7 256. 7 3. 5

T enszle properties Heat N0. 456

Hard- 0.2% U.'I.S., Elong, NIS/ Heat Treatment ness Y.S., K s.i. percent UTS R0 K s.i

1.-.- 1,600-12 Vrain A.C. 100 F.1 6 31:4 99. 5 142. 2 8.0

rs., 2 (1)+950 F.4 hrs., A.C 48. 2 209. 2 219 7. 5 3 (1)+950 F.-8 hrs, A.C 49. 4 219.0 227. 2 6. 5 4 25% C.R., 100 F., 16 hrs, A.W 162.0 170.8 6.0 5.." O.R., 100 F., 16 hrs., A.W 37. 2 172.1 184.1 2. 0 6 (4)+900 F., 1 hr., .A.C 46. 5 233. 2 233. 2 3. 5 7.-.. (4)+900 F., 4 hrs A C 49. 1 246.4 246. 4 3, 5 8 (4)+900 F., 8 hrs A C 50.5 257. 7 257. 7 2. 5 9 (4)+950 F., 1 hr A C 48. 9. 250.1 250.1 4.0 10... (4)+950 F., 4 hrs, A.C 50.7 259. 4 259. 4 4.0 11 (4)+950 F., 8 hrs, A.C 52 265. 3 265. 3 4.0

From the test results recorded in Table IV, it is seen that the steel after annealing at 1600 F. for 10 minutes followed by an air cooling and a sub-zero v cooling to.

100 F. for 16 hours followed by an air warming, exhibits a hardness of 32 R The tensile test indicates a level of 100,000 p.s.i. yield strength, 145,500 p.s.i. tensile strength and an: elongation of 8.5%, measured in 2". X-ray measurements and metallographic examinations have confirmed that the steel displayed a substantially completely martensitic structure. As would be expected with this low level of hardness, the steel possesses an excellent degree of formability as measured by. the bend test and the cup test. Thereafter, when the; steel is aged at a temperature of 950 F. for 4 hours followed by an air cooling the hardness is increased 15 points R and the corresponding yield strength is more than doubled. The steel also exhibits a tensile strength of about 220,000 p.s.i. and an elongation of about 6%. Of particular significance is the fact that this steel was also tested using a notched tensile bar in which; the; steel specimen being tested was notched to provide a sharp edged notch having a radius of 0.0007" at the base thereof. As thus tested, the ultimate tensile strength was measured and the ratio of the notched tensile strength to the ultimate tensile strength was compared. As set forth in Table IV, this ratio was almost a unity, thus indicating that the steel of the present inventionis highly notch ductile.

. are; illustrated after a, 50% cold working.

extremely low rate of work hardening, thus greatly contributing to the formability of the steel. As set forth in Table IV, the work hardening which was accomplished is effective for increasing the yield strength and the ultimate tensile strength without showing any highly adverse effect upon the ductility. Substantially similar results Thereafter the cold worked steel may be aged, and, depending upon the time at temperature, the steel will exhibit a correspondingly greater increase in the tensile properties, the yield strength varying between about 229,300 p.s.i. up to a maximum value of 249,700 p.s.i. having corresponding tensile strength of between about 229,300 up to about 250,000, the steel exhibiting about a 4% elongation as measured. over the 2" gauge length. When the steel is cold rolled 50% and thereafter aged at 950 F. for 4 hours, substantially similar results are obtained, it being noted that no adverse effect on the notch ductility of the steel is noted. Substantially similar tests were run on Heat 456 with substantially the same results being obtained.

It should be pointed out at this juncture that the mere comparison of the ductility readings et forth in Table IV, namely the percentage elongation measured over a 2-inch gauge length, is misleading as concerns the actual ductility exhibited by the steel of the present invention.

crease in hardness of 15.6 R units.

valid reduction of area measurements made for sheet materials, and since during tensile testing the specimens exhibited a high degree of necking, it is clear that all of the ductility has thus been localized. If valid figures could be obtained for the percentage reduction of area in the sheet materials, the comparable values would indicate that the steel exhibits an exceedingly high degree of duetility. Consequently, the mere comparison of the percentage elongation is not truly indicative of the ductility possessed by the steel of the present invention.

As stated hereinbefore, the steel of the present, invention is useful through a broad range of temperatures which run the gamut from the so-called cryogenic temperatures to temperatures approaching about 1000 F. Reference is respectfully directed to Table V which contains the results of tests conducted on Heat 319, these tests being performed both at room temperature and at 320 F.

ducted on the aged steel indicated a .02% yield strength of 183.9 K s.i., a .2% yield strength of 212.5 K s.i. a tensile strength of 219.9 K s.i. and an elongation, as measured over a 2-inch gauge length, of 14%. Thus it is clear that the commercial heat exhibited an outstanding response to provide an age hardened martensitic stainless steel having superior mechanical properties.

Additional annealed specimens from the hot rolled band were subjected to a cold working operation and were cold reduced 45% without difliculty. Thereafter the specimens were subzero cooled to l00 F. for a period of 16 hours, air warmed, aged at 950 for four hours and thereafter air cooled. As thus cold worked and heat treated, the steel exhibited a hardness of 50.8 R thus exhibiting a further increase in the hardness. Tensile tests revealed a 02% yield strength of 249.1 K s.i., a .2% yield strength of 263 K s.i., an ultimate strength of 266 K s.i. and a percent elongation as measured over From the test results recorded in Table V it is immediately apparent that the steel from Heat 319 possesses an outstanding combination of tensile properties when measured at room temperature. When measured at 320 F., these properties are outstanding, it being noted that there is no brittleness which occurs especially as measured by the percentage elongation; however, when this same steel is transformed other than by thermal heat treatments, i.e. by cold work, and is tested at room temperature, the tensile properties are at a substantially higher level. When tested at -320 F., the properties are substantially still higher yet without any adverse effect being noted in the ductility. Accordingly, it isquite clear that the steel of the present invention possesses an outstanding combination of properties even at the cryogenic temperatures. More important, however, the steel is not brittle and contains substantially the same ductility as it has at room temperature.

In commercial production, Heat No. 23932.0f the steel of the present invention was melted having the following analysis: carbon 0.021%, manganese 0.024%, silicon 0.08%, chromium 14.25%, nickel 3.40%, molybdenum 1.99%, titanium 0.35%, cobalt 15.44% and the balance substantially iron with incidental impurities. This heat was hot rolled into strip having a thickness of about 0.230". As hot rolled, samples were cut for hardness and tensile testing and thereafter the hot rolledband was annealed, pickled and cold rolled to approximately 0.129" in thickness. Hardness tests on the samples cut from the hot rolled band which were annealed at 1600 F. for minutes and air cooled revealed a hardness of R Specimens from the hot rolled strip after annealing at 1600 -F. for 25 minutes and air cooling, were subjected to a tensile test and the test results indicated a .02% yield strength of 51.6 K s.i., 'a 2% yield strength of 94.4 K s.i. and a tensile strength of 142 K s.i. The elongation measured over the 2-inch gauge length was 15.5%. Other specimens from the hot rolled band after annealing were subjected to a subzero cooling to -l00 F. for 16 hours followed by air warming and thereafter these specimens were aged at 950 F. for four hours and air cooled. As thus treated, these specimens had a hardness of 45.6 R indicating an in- Tensile tests conthe 2-inch gauge length of 4.0%. Again, the specimens exhibited a high degree of necking during the tensile test. Thus from the foregoing it is clearthat the commercial heats of this material exhibit an outstanding combination of mechanical properties, as substantiated by the test data set forth hereinbefore.

It has also been noted that the steel of the present invention exhibits an outstanding degree of formability. Bend tests on sheet material have been made and the steel in the annealed condition was bent over a pin having a diameter which measured twice the thickness of the sheet. No cracking was noted in said bend test. In addition, circular blanks measuring 4 in diameter were cut from said commercial Heat No. 23932 and were thereafter drawn into cups without any adverse effects, being noted. These cups measured 2 in diameter, thus illustrating a 42.7% reduction. This exc'ellent reduction, coupled with the fact that the steel of the present invention in its fully martensitic condition has an extremely low rate of work hardening, as witnessed by the fact that the steel can be cold worked to accomplish greater than a 50% reduction of cross sectional area of the strip without any adverse effects taking place, is a clear indication that the steel of the present invention has an outstanding degree of formability for a martensitic steel. Yet, through the proper heat treatment the steel of the present invention may be heat treated to develop an outstanding combination of mechanical properties.

The steel of the present invention, because of low carbon content, has an excellent degree of corrosion resistance to the various media and more importantly, if the steel is balanced so that the cobalt content is near the lower limit of about 3% and the nickel content is increased to near its upper limit of about 7%, the steel, while not attaining the high level of mechanical properties, will nonetheless possess an excellent degree of resistance to stress corrosion. This is a very important factor in some applications where some of the strength characteristics can be sacrificed in order to take advantage of the stress corrosion properties of the steel. As amply illustrated-by the test data contained in the foregoing specification, the steel has an outstanding degree of ductility and possesses an extremely attractive ratio of notch tensile strength to ultimate tensile strength, thus making the steel of the present invention particularly applicable for use in aircraft designed for flights at supersonic speeds.

There are no special skills required to practice this invention, nor any critical heat treatments utilized to vary the internal chemistry of the steel. The steel can be readily Welded, and, since it is a martensitic steel, the application of a simple aging heat treatment is effective for imparting a high level of mechanical properties thereto without'the decrease of warpage through a high temperature heat treatment.

I claim: a 1. An age hardenable martensitic stainless steel consisting essentially of traces to about 0.05% carbon, traces to about 0.25 manganese, traces to about 0.25% silicon, from about 10% to about 18% chromium, from about 0.1% to about 7% nickel, from about 3% to about 16% cobalt, from about 0.1% to about 8% molybdenum, from about 0.1% to about 1.3% titanium, and the balance iron with incidental impurities.

2. An age hardenable martensitic stainless steel consisting essentially of traces to about 0.03% carbon, traces to about 0.15% manganese, traces to about 0.15% silicon, from about 12% to about 15% chromium, from about 2% to about 7% nickel, from about 10% to about 16% cobalt, from about 1% to about 6% molybdenum, from about 0.1% to about 1.0% titanium, and the balance iron with incidental impurities.

3. An age hardenable martensitic stainless steel consisting essentially of traces to about 0.05% carbon, traces to about 0.25% manganese, traces to about 0.25% silicon, from about 10% to about 18% chromium, from about 0.1% to about 7% nickel, from about 3% to about 16% cobalt, from about 0.1% to about 8% molybdenum, from about 0.1% to about 1.3% titanium, and the balance essentially iron with incidental impurities and which is characterized by exhibiting a marten-sitic structure having a hardness in the annealed condition of 35 R maximum resulting from balancing the composition of the alloy as respects the ferrite-forming elements and the austenite-forming elements in accordance with the area AcBCDaA of the diagram of the accompanying drawing.

4. An age hardenable martensitic stainless steel con-sisting essentially of traces to about 0.03% carbon, traces to about 0.15% manganese, traces to about 015% silicon, from about 12% to about 15% chromium, from about 2% to about 7% nickel, from about 10% to about 16% cobalt, from about 1% to about 6% molybdenum, from about 0.1% to about 1.0% titanium, and the balance essentially iron with incidental impurities and which is characterized by exhibiting a martensitic structure having a hardness in the annealed condition of 35 R maximum resulting from balancing the composition of the alloy as respects the ferrite-forming elements and the austeniteforming elements in accordance with the area AcbaA of the diagram of the accompanying drawing.

5. An age hardened martensitic high strength stainless steel characterized by exhibiting a minimum hardness of 45 R after transforming the steel to martensite followed by aging the steel at a minimum temperature of 900 F. for a time period of at least 4 hours and consisting essentially of traces to about 0.05 carbon, traces to about 0.25% manganese, traces to about 0.25% silicon, from about 10% to about 18% chromium, from about 0.1% to about 7% nickel, from about 7% to about 16% cobalt, from about 0.1% to about 8% molybdenum, from about 0.1% to about 1.3% titanium and the balance essentially iron with incidental impurities.

6. An age hardened martensitic high strength stainless steel characterized by exhibiting a minimum hardness of 45 R after transforming the steel to martensite followed by aging the steel at ,a minimum temperature of 900 F.

for a time period of at least 4 hours and consisting cssentially of traces to about 0.03% carbon, traces to about 0.15 manganese, traces to about 0.15% silicon, from about 12% to about 15 chromium, from about 2% to about 7% nickel, from about 10% to about 16% cobalt, from about 1% to about 6% molybdenum, from about 0.1% to about 1.0% titanium and the balance essentially iron with incidental impurities.

7. An-age hardenable martensitic stainless steel consisting essentially of traces to about 0.05 carbon, traces to about 0.25% manganese, traces to about 0.25 silicon, from about 10% to about 18% chromium, from about 0.1% to about 7% nickel, from about 7% to about 16% cobalt, from about 0.1% to about 8% molybdenum, from about 0.1% toabout 1.3% titanium, and the balance iron with incidental impurities.

8. A martensitic high strength stainless steel article of manufacture which is characterized by having abalanced composition of austenite formers and ferrite formers in accordance with the .area AcBCDaA of the ac companying drawing, which exhibits a minimum hardness of 45 R after aging for at least four hours at a temperature within the range between 900 F. and 1050 F. and which has a composition that includes from traces to about 0.05% carbon, from traces to about 0.25 manganese, from traces to about 0.25 silicon, from about 10% to about 18% chromium, from about 0.1% to about 7% nickel, from about 7% to about 16% cobalt, from about 0.1% to about 8% molybdenum, from about 0.1% to about 1.3% titanium and the balance essentially iron with incidental impurities.

9. A martensitic high strength stainless steel article of manufacture which is characterized by having a balanced composition of austenite formers and ferrite formers in accordance with the area AcBCDaA of the accompanying drawing which exhibits a minimum hardness of 45 R. after aging for at least four 'hours at a temperature within the range between 900 F. and 1050 F. and which has a composition that includes from traces to about 0.03% carbon, from traces to about 0.15 manganese, from traces to about 0.15% silicon, from about 12% to about 15 chromium, from about 2% to about 7% nickel, from about 10% to about 16% cobalt, from about 1% to about 6% molybdenum, from about 0.1% to about 1.0% titanium and the balance essentially iron with incidental impurities. I

10. A martensitic high strength stainless steel article of manufacture characterized by having a composition which includes from traces to about 0.05% carbon, from traces to about 0.25 manganese, from traces to about 0.25% silicon, from 10% to about 18% chromium, from about 0.1% to about 7% nickel, from about 3% to about 16% cobalt, from about 0.1% to about 8% molybdenum, from about 0.1% to about 1.3% titanium and the balance essentially iron with incidental impurities, and the article exhibiting a hardness of 49 R minimum when the article is subzero cooled to a temperature not lower than --100 F. and thereafter aged at a temperature within the range between 900 F. and 1050 F. and the steel from which said article is manufactured has been cold worked to effect a reduction in the cross sectional area of up to 50% prior to the formation of said article.

References Cited by the Examiner UNITED STATES PATENTS 2,496,248 1/1950 Jennings 128.9 2,793,948 5/1957 Wagner 75-1288 FOREIGN PATENTS 796,733 6/1958 Great Britain.

DAVID L. RECK, Primary Examiner.

ROGER L. CAMPBELL, Examiner. 

1. AN AGE HARDENABLE MARTENSITIC STAINLESS STEEL CONSISTING ESSENTIALLY OF TRACES TO ABOUT 0.05% CARBON, TRACES TO ABOUT 0.25% MANGANESE, TRACES TO ABOUT 0.25% SILICON, FROM ABOUT 10% TO ABOUT 18% CHROMIUM, FROM ABOUT 0.1% TO ABOUT 7% NICKEL, FROM ABOUT 3% TO ABOUT 16% COBALT, FROM ABOUT 0.1% TO ABOUT 8% MOLYBDENUM, FROM ABOUT 0.1% TO ABOUT 1.3% TITANIUM, AND THE BALANCE IRON WITH INCIDENTAL IMPURITIES. 